This invention relates to information storage systems and in particular to information storage systems having a single magnetic head transducer for recording, playback and erasing of information.
In recording or reading information on a moving information storage media, such as a magnetic recording disc, relative motion between a magnetic head transducer used for reading and writing information on the storage media on which information is written on and read from is required. The relative velocity between the head and media interface may be, for example, 550 IPS for video and high density digital applications. To achieve maximum performance, an interface of intimate contact is provided between the head and media surface without destruction or excessive wear of the magnetic flux-responsive coating on the surface.
As contact pressure between the head and media is increased to improve performance, both media and head wear are increased. The problem is particularly severe in video applications and compounded with the medias utilized for storing single video frames on separate tracks of a magnetic disc wherein one track at a time is continuously in contact with the head to provide a continuous stationary display of a single frame. In only five minutes of playtime revolving at 3600 RPM, for example, a track on the disc is 18000 times by the head; the wear by-products are so hard and abrasive that the same materials are commonly used as lapping compounds.
To prevent failure caused by contact between the head and disc, lubricated surfaces and/or air film separation have been used. However, any separation between the head and disc caused by such lubrication fluid or air film imposes a loss of signal and hence performance. A head/disc separation equal to one wave length could cause about 54.6 db loss in the output of the replay head. Since it is desired to record wave lengths that approach 70 microinches (0.0017 mm), the playback head voltage is reduced to 50% by only 7.7 microinches (0.0002 mm) of separation. On the other hand, as discussed above, reduction of separation to meet desired performance would cause the interface to be destroyed within a few seconds.
Prior art solutions to the head-to-disc interface problem have generally been of two types: flying heads in conjunction with rigid hard-surface discs and heads having large surface areas buried in soft flexible "floppy" discs. Flying head discs are very expensive and require complicated and expensive recording/playback systems. A flexible or so-called "floppy" disc reduces the handling and cost problems realized in the flying head rigid-disc systems. Some record/read heads for "floppy" discs are relatively large to provide an interface comprised of a large contoured head buried in the soft flexible media. The large record/read head surface area distributes the force per unit area to reduce media wear and separation loss. As the media is moved past the head, however, air collects between the head and disc surface to form an air film. The thickness of this air film is a function of head and media surface finishes, media stiffness, head-media penetration, head size, head surface contour, viscosity of the air and disc-head relative velocity. Because of these restraints, most flexible or "floppy" disc applications are limited to slow speed, low bandwidth digital computer applications or voice recording systems.
To erase information from selected recording tracks of the disc a special erase head, which is wider than a normal recording head, is typically used. The use of a wide head ensures that the entire breadth of the recording track is erased and all remnants of the information signal removed. Tunnel erase and straddle erase methods are also typically used, but are expensive and require complicated manufacturing processes.
Alternatively, when a single head is used for recording, playback and erasure, complete erasure of an entire recording track and the edges thereof is difficult to achieve because portions of the signal may be recorded outside of the region where the head can effectively erase when positioned on the centerline of the track. This is of course due to small excursions of the head from the center line during the recording process as well as the interchanging of discs and disc cartridge and drive manufacturing tolerances.
For example, if the head is 8 mils (0.203 mm) wide, it can effectively erase information in an 8 mil-wide region, 4 mils (0.11 mm) on either side of the center line. If the recording system allows excursions of up to 1 mil (0.025 mm) in either direction from the center line, information may then be recorded in a 10 mil (0.254 mm)-wide region, 5 mils (0.127 mm) on either side of the center line. Thus portions of the information signal are recorded in the 1 mil-wide bands between 4 and 5 mils on each side of the centerline and can not be effectively erased by an 8 mil-wide recording head.